Changes in action potential and beta-adrenergic effects of the circular muscle of postpartum rat uterus.

Spike potential dominated in the circular muscle of postpartum rat uterus during the period between 0 and 15 hr after the delivery of the first newborn. During the postpartum period ranging between 20 and 48 hr, the plateau potential was dominant. Application of 10(-9) M isoprenaline strongly depressed the contraction during early postpartum period (0-15 hr), and the depressant effect was much smaller thereafter. In vivo treatment of postparturient rats with estradiol-17 beta (50 micrograms) or progesterone (50 micrograms) for 2 days did not alter the postpartum change in action potential or the effect of isoprenaline. The postpartum changes in muscle properties mentioned above were prevented in the distended portion of uterus, when several fetuses and placentas were artificially kept inside the uterus for 2 days, while other fetuses were delivered out. The hormonal influences on the circular muscle of postpartum rat uterus were discussed in view of the above experimental findings.